1. Field of the Invention
The present invention relates to light-emitting diodes, and more particularly, to light-emitting diodes having anti-reflection coatings and to a method of making same.
2. Description Relative to the Prior Art
As text and image information generation and transmission becomes increasingly available in digitized, electronic form, the need for producing hard copies of such information directly with low cost, high speed printers increases. An attractive approach for satisfying this need is in the provision of a high-density, compact, linear array of LEDs used in electronic/optical printers. Besides the obvious advantage of compactness and increased density of light sources, the LED array system promises the advantage of being more reliable and less expensive for such use.
LED arrays are well known in the prior art and are typically used in printheads for electrophotographic copiers or the like. For example, see U.S. Pat. No. 4,885,597 the contents of which are incorporated by this reference. Such a printhead array comprises a row of uniformly spaced LED light sources that can be individually energized to expose a photoreceptor or other image-receiving medium to produce an image pattern. A typical LED printhead array of this type for standard DIN A4 paper dimensions would be about 216 millimeters long and the individual light sources are very small and very closely spaced, e.g. as many as 400 or more sites per linear inch. A complete printhead array comprises a number of individual array chips, each being typically less than 10 millimeters long, which are mounted in endwise relation to one another to provide the full length printhead array.
Important to printing applications is the stringent requirement placed on the uniformity of the intensity of light emission from the LED arrays used in the electronic/optical printer. In display applications for LED arrays, uniformity is not nearly as critical as in a printer because the eye is a logarithmic detector and is relatively insensitive to light intensity variations. However, in printing applications which involve relative movement of a photosensitive material past a linear array of LEDs, small intensity variations will manifest themselves in rasterbanding effects.
One factor associated with intensity variations from LED to LED is temperature. In order to obtain sufficient light output from the LED's to properly expose the recording medium, relatively high levels of current are provided to the printhead. As the temperature of the LED's is increased through production operations, efficiency of the LED's on production of light falls and additional current is required to offset this.
To this end, various proposals have been made to form more efficient LED's. In U.S. Pat. No. 4,617,192, the advantages of anti-reflection coatings on LED's are noted. As taught in this patent, an anti-reflection coating on an LED insures greater output of radiation from the LED, particularly where there is judicious selection of the coating material and its thickness. According to this patent, for a single layer anti-reflection coating with air as one of the optical mediums, optimum results can be obtained if the index of refraction of the anti-reflection coating is the square root of the index of refraction of the layer beneath the coating. Regarding thickness, the patent notes a relationship exists between thickness of the anti-reflection layer with wavelength of the radiation being transmitted.
U.S. Pat. No. 4,495,514 also notes the importance that thickness plays in an anti-reflection coating to minimize Fresnel reflections at the device-air interface.
In fabricating LED's, it is common practice to provide a dielectric mask over an n-type layer in which the p-n junction is formed. This mask is selectively etched away to form a window in which a p-type region is diffused into the n-type layer. In the course of diffusion down into the n-type layer there is also lateral diffusion of the p-type region into areas beneath the dielectric mask. Thereafter, an anti-reflection coating is applied over the window to improve light output efficiency. While the prior art recognizes that it is desirable to adjust the thickness of the anti-reflection coating over the window for optimum light output and recognizes that light is also output from the diffused areas beneath the mask, the presence of the mask is effectively ignored by considering same to be transmissive.
The inventors herein have noted that there is a fall-off in light output from the mask areas. In seeking to create more effective LED's, the inventors have investigated the reasons for this fall-off and determined that the mask may not be simply thought of as being transmissive but that a relationship exists between the combined thickness of mask layer and anti-reflection coating layer and that these can be optimized as well to significantly improve light output from this region of the LED to enhance light output by the LED.